The prior art contains examples of power steering systems utilizing reaction chambers which act on steering control valves in such a way as to simulate response to steering at various speeds of a vehicle. For example, such an arrangement is shown in German Patent OS No. 32 20 922. In that patent, the hydraulic reaction force is varied by means of a control mechanism comprising a speed responsive throttle which regulates or proportions the manual steering force at the steering wheel.
Usually booster steering systems are designed so that characteristic lines on a graph of their steering control speeds do not run precisely toward the zero point of the graph but are horizontally tangent to it. Accordingly, there is an initial steering reaction response which must be overcome by manual force at the steering wheel before the actual booster steering force commences. This is a requirement for good stability in straight-ahead travel.
Booster systems utilizing pressure in reaction chambers provide means for varying such pressure responsive to vehicle speed, and in such a manner that the characteristic lines for varying speeds become steeper with decreasing speed of the vehicle. Thus, initial steering reaction response, which may be called a centering force, causes at low speed small servomotor pressures and manual force at the steering wheel is disproportionately low for steeper characteristic lines. The simulated steering response achieved seems to be weak and uncertain because of low booster pressure due to slower speeds.
In servosteering systems which provide booster pressure, the manual steering force is limited in its magnitude to a maximum by means of a so-called "cut-off", namely, a pressure reduction valve which limits the pressure difference independently of a particular vehicle speed operating on a slanted characteristic line of a graph showing manual steering force vs. servomotor pressure. However, the steering seems to require more manual effort due to reaction chamber pressure in the range of medium or high speeds of the vehicle when steering in tight curves.